Abstract
Abstract. Dissolved organic carbon (DOC) concentrations – mainly of terrestrial origin – are increasing worldwide in inland waters. Heterotrophic bacteria are the main consumers of DOC and thus determine DOC temporal dynamics and availability for higher trophic levels. Our aim was to study bacterial carbon (C) turnover with respect to DOC quantity and chemical quality using both allochthonous and autochthonous DOC sources. We incubated a natural bacterial community with allochthonous C (13C-labeled beech leachate) and increased concentrations and pulses (intermittent occurrence of organic matter input) of autochthonous C (phytoplankton lysate). We then determined bacterial C consumption, activities, and community composition together with the C flow through bacteria using stable C isotopes. The chemical analysis of single sources revealed differences in aromaticity and low- and high-molecular-weight substance fractions (LMWS and HMWS, respectively) between allochthonous and autochthonous C sources. Both DOC sources (allochthonous and autochthonous DOC) were metabolized at a high bacterial growth efficiency (BGE) around 50%. In treatments with mixed sources, rising concentrations of added autochthonous DOC resulted in a further, significant increase in bacterial DOC consumption of up to 68% when nutrients were not limiting. This rise was accompanied by a decrease in the humic substance (HS) fraction and an increase in bacterial biomass. Changes in DOC concentration and consumption in mixed treatments did not affect bacterial community composition (BCC), but BCC differed in single vs. mixed incubations. Our study highlights that DOC quantity affects bacterial C consumption but not BCC in nutrient-rich aquatic systems. BCC shifted when a mixture of allochthonous and autochthonous C was provided simultaneously to the bacterial community. Our results indicate that chemical quality rather than source of DOC per se (allochthonous vs. autochthonous) determines bacterial DOC turnover.
Highlights
The major organic carbon (C) pool in aquatic ecosystems is dissolved organic carbon (DOC; Cole et al, 2007)
Dissolved organic carbon (DOC) consists of a heterogeneous mixture of different C compounds that vary in chemical quality and include both autochthonous DOC (e.g., C released by phytoplankton and macrophytes) and allochthonous C originating from terrestrial areas surrounding aquatic ecosystems
We revealed that bacterial decomposition of a mixture of both allochthonous and autochthonous DOC sources changes with quantity of autochthonous C and bacterial community composition (BCC) changes with chemical quality
Summary
The major organic carbon (C) pool in aquatic ecosystems is dissolved organic carbon (DOC; Cole et al, 2007). DOC consists of a heterogeneous mixture of different C compounds that vary in chemical quality and include both autochthonous DOC (e.g., C released by phytoplankton and macrophytes) and allochthonous C originating from terrestrial areas surrounding aquatic ecosystems. Allochthonous C inputs to aquatic ecosystems can vary in time and quality, e.g., particulate or dissolved forms (Carpenter et al, 2005). In forested watersheds, substantial amounts of leaves can fall into lakes and dissolve into the water column K. Attermeyer et al.: Enhanced bacterial decomposition not accompanied by community changes and Hasler, 1976; France, 1995; Vander Zanden and Gratton, 2011)
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